Polishing apparatus

ABSTRACT

A polishing apparatus which can remove slurry which has entered into a gap between an elastic membrane for pressing a substrate such as a wafer and a retaining ring is disclosed. The polishing apparatus includes a top ring which has an elastic membrane configured to form a pressure chamber for pressing the substrate against a polishing pad, and a retaining ring disposed around the elastic membrane and configured to press the polishing pad, a top ring rotating device configured to rotate the top ring about an axis of the top ring, and a cleaning brush configured to be brought into contact with a peripheral portion of a lower surface and an outer circumferential surface of the elastic membrane.

CROSS REFERENCE TO RELATED APPLICATION

This document claims priority to Japanese Patent Application Number 2013-146537 filed Jul. 12, 2013, the entire contents of which are hereby incorporated by reference.

BACKGROUND

A polishing apparatus for polishing a surface of a wafer generally includes a polishing table for supporting a polishing pad having a polishing surface, and a top ring for holding the wafer. While rotating the polishing table and the top ring, the top ring presses the wafer against the polishing surface of the polishing pad under a predetermined pressure to bring the wafer into sliding contact with the polishing surface, thereby polishing a surface of the wafer. In chemical mechanical polishing (CMP), a polishing liquid (slurry) is supplied onto the polishing surface during polishing.

In such polishing apparatus, if the relative pressing force applied between the wafer and the polishing surface of the polishing pad is not uniform over the entire surface of the wafer, then the surface of the wafer is polished insufficiently or excessively in different regions thereof. It has been customary to uniformize the pressing force applied to the wafer by providing a pressure chamber formed by an elastic membrane at a lower portion of the top ring and supplying the pressure chamber with a fluid such as air to press the wafer under a fluid pressure through the elastic membrane.

During polishing of the wafer, a frictional force acts between the wafer and the polishing pad. In order to prevent the wafer from disengaging from the top ring by the frictional force, a retaining ring for retaining a position of the wafer is arranged. This retaining ring is disposed so as to surround the elastic membrane and the wafer, and is configured to press the polishing pad around the wafer.

When the wafer is polished in the presence of the slurry, the slurry is attached to the elastic membrane of the top ring. The slurry attached to the elastic membrane is eventually solidified, and the solid slurry may fall onto the polishing surface and cause a scratch on the surface of the wafer. Therefore, in the conventional polishing apparatus, a fluid is ejected to the elastic membrane to remove the slurry. However, it is difficult to remove the solidified slurry from the elastic membrane by ejecting the fluid, and especially it has been difficult to remove the slurry present in a gap between the retaining ring and the elastic membrane.

SUMMARY OF THE INVENTION

It is therefore an object to provide a polishing apparatus which can remove slurry which has entered into a gap between an elastic membrane for pressing a substrate such as a wafer and a retaining ring.

Embodiments, which will be described below, relate to a polishing apparatus, and more particularly to a polishing apparatus for polishing a surface of a substrate such as a wafer.

In order to achieve the above object, in an embodiment, there is provided a polishing apparatus, including: a polishing table configured to support a polishing pad; a top ring which has an elastic membrane configured to form a pressure chamber for pressing a substrate against the polishing pad, and a retaining ring disposed around the elastic membrane and configured to press the polishing pad; a top ring rotating device configured to rotate the top ring about an axis of the top ring; and a cleaning brush configured to be brought into contact with a peripheral portion of a lower surface and an outer circumferential surface of the elastic membrane.

In an embodiment, the polishing apparatus further includes a cleaning liquid supply nozzle configured to supply a jet of a cleaning liquid to the cleaning brush.

In an embodiment, the polishing apparatus further includes a cleaning brush movement mechanism configured to move the cleaning brush between a cleaning position below the top ring and a standby position outside of the top ring.

In an embodiment, the cleaning brush is brought into contact with an inner circumferential surface of the retaining ring.

In an embodiment, the cleaning brush is brought into contact with a lower surface of the retaining ring.

In an embodiment, the cleaning brush is a first cleaning brush, and a second cleaning brush configured to be brought into contact with a zone more inner than the peripheral portion in the lower surface of the elastic membrane is further provided.

According to the above-described embodiments, the cleaning brush is brought contact with the peripheral portion of the lower surface and the outer circumferential surface of the elastic membrane, and thus the polishing liquid (slurry) present in the gap between the elastic membrane and the retaining ring can be removed. Therefore, it is possible to prevent a solidified polishing liquid from being deposited into the gap between the elastic membrane and the retaining ring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a polishing apparatus according to an embodiment;

FIG. 2 is a cross-sectional view showing a top ring having a plurality of air bags capable of pressing plural zones of a wafer independently;

FIG. 3 is a view showing the top ring located in a substrate transfer position;

FIG. 4 is a side view showing a cleaning assembly;

FIG. 5 is a top plan view of the cleaning assembly;

FIG. 6 is an enlarged view showing a cleaning brush and part of the top ring;

FIGS. 7A and 7B are schematic views showing the positional relationship between the cleaning assembly and the top ring;

FIG. 8 is a view showing an embodiment in which a cleaning nozzle for supplying a cleaning liquid to a gap between a membrane and a retaining ring is provided;

FIG. 9 is a side view showing another embodiment of the cleaning assembly;

FIG. 10 is a top plan view of the cleaning assembly shown in FIG. 9;

FIG. 11 is a view illustrating a state in which the relative position of the cleaning brush with respect to a second brush arm is changed;

FIG. 12 is a view showing an embodiment in which a tilt mechanism for tilting the cleaning brush with respect to the vertical direction is provided;

FIG. 13 is a side view showing still another embodiment of the cleaning assembly;

FIG. 14 is a top plan view showing still another embodiment of the cleaning assembly;

FIG. 15 is a side view showing the cleaning assembly shown in FIG. 14;

FIGS. 16A and 16B are schematic views showing the positional relationship between the cleaning assembly and the top ring;

FIG. 17 is a side view showing an embodiment in which the second cleaning brush shown in FIGS. 14 and 15 is incorporated in the cleaning assembly shown in FIG. 9; and

FIG. 18 is a side view showing an embodiment in which the second cleaning brush shown in FIGS. 14 and 15 is incorporated in the cleaning assembly shown in FIG. 13.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to drawings.

FIG. 1 is a view showing a polishing apparatus according to an embodiment. As shown in FIG. 1, the polishing apparatus includes a polishing table 10, and a top ring (substrate holding unit) 1 for holding a substrate, such as a wafer, as an object to be polished and pressing the substrate against a polishing pad 20 on the polishing table 10.

The polishing table 10 is coupled via a table shaft 10 a to a motor (not shown) disposed below the polishing table 10. Thus, the polishing table 10 is rotatable about the table shaft 10 a. The polishing pad 20 is attached to an upper surface of the polishing table 10. A surface 20 a of the polishing pad 20 serves as a polishing surface for polishing a wafer W. A polishing liquid supply nozzle 62 is provided above the polishing table 10 to supply a polishing liquid (slurry) Q onto the polishing pad 20 on the polishing table 10.

The top ring 1 is connected to a top ring shaft 65, which is vertically movable relative to a top ring head 64 by a vertically moving mechanism 81. The vertical movement of the top ring shaft 65 enables the top ring 1 in its entirety to move upward and downward and enables positioning of the top ring 1 with respect to the top ring head 64. A rotary joint 82 is mounted to the upper end of the top ring shaft 65.

The vertically moving mechanism 81 for vertically moving the top ring shaft 65 and the top ring 1 includes a bridge 84 for rotatably supporting the top ring shaft 65 through a bearing 83, a ball screw 88 mounted to the bridge 84, a support pedestal 85 supported by support posts 86, and a servomotor 90 provided on the support pedestal 85. The support pedestal 85, which supports the servomotor 90, is fixed to the top ring head 64 through the support posts 86.

The ball screw 88 includes a screw shaft 88 a coupled to the servomotor 90 and a nut 88 b that engages with the screw shaft 88 a. The top ring shaft 65 is vertically movable in unison with the bridge 84. Therefore, when the servomotor 90 is set in motion, the bridge 84 moves vertically through the ball screw 88, so that the top ring shaft 65 and the top ring 1 move vertically.

Further, the top ring shaft 65 is connected to a rotary sleeve 66 by a key (not shown). The rotary sleeve 66 has a timing pulley 67 at its outer circumferential portion. A top ring rotating motor (top ring rotating device) 68 is fixed to the top ring head 64. The timing pulley 67 is operatively coupled to a timing pulley 70, provided on the top ring rotating motor 68, through a timing belt 69. Therefore, when the top ring rotating motor 68 is set in motion, the rotary sleeve 66 and the top ring shaft 65 are rotated in unison with each other through the timing pulley 70, the timing belt 69, and the timing pulley 67, thus rotating the top ring 1. The top ring head 64 is supported by a top ring head shaft 80, which is rotatably supported by a frame (not shown). The polishing apparatus further includes a controller 50 for controlling respective devices in the apparatus including the top ring rotating motor 68 and the servomotor 90.

The top ring 1 is configured to hold the wafer W on its lower surface. The top ring head 64 is configured to be pivotable about the top ring head shaft 80. Thus, the top ring 1, which holds the wafer W on its lower surface, is moved from a position at which the top ring 1 receives the wafer W (substrate transfer position) to a position above the polishing table 10 by a pivotal movement of the top ring head 64.

Polishing of the wafer W is performed as follows. While the top ring 1 and the polishing table 10 are rotated individually, the polishing liquid Q is supplied onto the polishing pad 20 from the polishing liquid supply nozzle 62 provided above the polishing table 10. In this state, the top ring 1 is lowered and then presses the wafer W against the polishing surface 20 a of the polishing pad 20. The wafer W is brought in sliding contact with the polishing surface 20 a of the polishing pad 20, so that a surface of the wafer W is polished.

FIG. 2 is a cross-sectional view showing the top ring 1 having a plurality of air bags capable of pressing plural zones of the wafer W independently. The top ring 1 has a top ring body 41 coupled to the top ring shaft 65 through a universal joint 40, and a retaining ring 42 provided below the top ring body 41.

A flexible membrane (or an elastic membrane) 46 to be brought into contact with the wafer W, and a chucking plate 47 that holds the membrane 46 are disposed below the top ring body 41. Four pressure chambers (i.e., air bags) C1, C2, C3, and C4 are provided between the membrane 46 and the chucking plate 47. The pressure chambers C1, C2, C3, and C4 are formed by the membrane 46 and the chucking plate 47. The central pressure chamber C1 has a circular shape, and the other pressure chambers C2, C3, and C4 have an annular shape. These pressure chambers C1, C2, C3, and C4 are in a concentric arrangement.

Pressurized gas (i.e., pressurized fluid), such as pressurized air, is supplied into the pressure chambers C1, C2, C3, and C4 from the pressure regulator 100 through fluid passages F1, F2, F3, and F4, respectively. The pressures in the pressure chambers C1, C2, C3, and C4 can be changed independently to thereby independently regulate polishing pressures applied to the corresponding four zones of the wafer W, i.e., a central zone, an inner intermediate zone, an outer intermediate zone, and a peripheral zone.

A pressure chamber C5 is formed between the chucking plate 47 and the top ring body 41. The pressurized gas is supplied into the pressure chamber C5 from the pressure regulator 100 through a fluid passage F5, or a negative pressure is formed in the pressure chamber C5. With this operation, the chucking plate 47 and the membrane 46 in their entirety can move vertically. The peripheral end portion of the wafer W is surrounded by the retaining ring 42 so as to prevent the wafer W from coming off the top ring 1 during polishing. The membrane 46 has an opening in a portion that forms the pressure chamber C3, so that the wafer W can be held by the top ring 1 via the vacuum suction by producing a vacuum in the pressure chamber C3. Further, the wafer W can be released from the top ring 1 by supplying nitrogen gas, clean air, or the like into the pressure chamber C3.

An annular rolling diaphragm 49 is provided between the top ring body 41 and the retaining ring 42. A pressure chamber C6 is formed in the rolling diaphragm 49. The pressure chamber C6 is coupled to the above-described pressure regulator 100 through a fluid passage F6. The pressure regulator 100 supplies the pressurized gas into the pressure chamber C6, so that the retaining ring 42 presses the polishing pad 22.

The pressurized gas from the pressure regulator 100 is supplied into the pressure chambers C1 to C6 through the rotary joint 82 (see FIG. 1) and the fluid passages F1, F2, F3, F4, F5, and F6, respectively. Further, it is also possible to form a negative pressure in the pressure chambers C1 to C6 by the pressure regulator 100. Furthermore, the pressure chambers C1 to C6 are also coupled to vent valves (not shown), respectively, so that the pressure chambers C1 to C6 can be ventilated to the atmosphere.

The controller 50 is configured to determine target values of the pressures in the pressure chambers C1, C2, C3, and C4 based on the progress of polishing at respective zones of the wafer W which are located at positions corresponding to the pressure chambers C1, C2, C3, and C4. The controller 50 sends command signals to the pressure regulator 100 and controls the pressure regulator 100 such that the pressures in the pressure chambers C1, C2, C3, and C4 match the above-described target values, respectively. The top ring 1 having the plural pressure chambers can polish the wafer W uniformly because the pressure chambers can independently press the respective zones of the surface of the wafer W against the polishing pad 20 according to the progress of polishing.

FIG. 3 is a view showing the top ring 1 located in the substrate transfer position which is lateral to the polishing table 10. The top ring 1 shown in FIG. 3 is in a state after the top ring 1 has transferred the wafer to a substrate transfer device such as a pusher and a transfer robot (not shown). As shown in FIG. 3, a cleaning assembly 110 for cleaning the lower surface of the top ring 1 is provided at the substrate transfer position. This cleaning assembly 110 is fixed to a frame (not shown) of the polishing apparatus.

FIG. 4 is a side view showing the cleaning assembly 110, and FIG. 5 is a top plan view of the cleaning assembly 110. The cleaning assembly 110 includes a cleaning brush 111 and a brush arm 114 for holding the cleaning brush 111. The cleaning brush 111 comprises a number of bristles having the same length and extending vertically. These bristles are made of nylon, for example, and the diameter of each bristle is 0.1 mm to 0.5 mm. The cleaning brush 111 is arranged so that its upper end is brought into contact with a peripheral portion of a lower surface of the membrane (elastic membrane) 46 and a lower surface of the retaining ring 42. A lower end of the cleaning brush 111 is fixed to the brush arm 114.

A tip (upper end) of the cleaning brush 111 forms a horizontal plane. The cleaning brush 111 shown in FIGS. 4 and 5 is separated into three bundles arrayed along a radial direction of the top ring 1. However, the present invention is not limited to this arrangement. For example, the cleaning brush 111 is not separated into a plurality of bundles, but may be arrayed simply along the radial direction of the top ring 1.

The cleaning assembly 110 further includes a cleaning liquid supply nozzle 117 for supplying a jet of a cleaning liquid to the cleaning brush 111. This cleaning liquid supply nozzle 117 is held by a nozzle holder 118 fixed to the brush arm 114 and is arranged adjacent to the cleaning brush 111. The cleaning liquid supply nozzle 117 is inclined upwardly with respect to the horizontal direction, and is arranged to apply the jet of the cleaning liquid onto the upper part of the cleaning brush 111. An example of the cleaning liquid used is pure water. An angle of the cleaning liquid supply nozzle 117 with respect to the horizontal direction and/or the vertical direction may be variable.

The cleaning assembly 110 further includes a cleaning brush movement mechanism 120 for moving the cleaning brush 111 and the cleaning liquid supply nozzle 117. This cleaning brush movement mechanism 120 includes an air cylinder 121 as an actuator, and a link 124 connected to a piston rod 121 a of the air cylinder 121 through a joint 123. The brush arm 114 is rotatable about a pivot 126. When the piston rod 121 a of the air cylinder 121 is moved, the brush arm 114 is rotated by a predetermined angle about the pivot 126. Thus, the cleaning brush 111 and the cleaning liquid supply nozzle 117 fixed to the brush arm 114 are moved integrally.

FIG. 6 is an enlarged view showing the cleaning brush 111 and part of the top ring 1. As shown in FIG. 6, part of the cleaning brush 111 extends into a gap between the membrane 46 and the retaining ring 42, and is thus brought into contact with an outer circumferential surface of the membrane 46 and an inner circumferential surface of the retaining ring 42. At the same time, the cleaning brush 111 is brought into contact with a peripheral portion of the lower surface of the membrane 46 and an inner peripheral portion of the lower surface of the retaining ring 42.

FIGS. 7A and 7B are schematic views showing the positional relationship between the cleaning assembly 110 and the top ring 1 in the substrate transfer position. When the piston rod 121 a of the air cylinder 121 moves forward, the brush arm 114 is rotated by a predetermined angle, and thus the cleaning brush 111 is moved to a cleaning position below the top ring 1 shown in FIG. 7A. When the piston rod 121 a of the air cylinder 121 moves backward, the brush arm 114 is rotated in the opposite direction, and thus the cleaning brush 111 is moved to a retreated position outside of the top ring 1 shown in FIG. 7B.

Cleaning of the top ring 1 by the cleaning assembly 110 is carried out when the top ring 1 is in the substrate transfer position outside of the polishing table 10. More specifically, the top ring 1 holding the wafer is moved to the substrate transfer position by rotation of the top ring head shaft 80, and the wafer is transferred to the substrate transfer device (not shown) at the substrate transfer position. Then, the top ring 1 is rotated about its axis by the top ring rotating motor (top ring rotating device) 68. Further, the air cylinder 121 is actuated to move the cleaning brush 111 to the cleaning position below the top ring 1 (see FIG. 7A). In this state, the cleaning liquid is jetted toward the cleaning brush 111 from the cleaning liquid supply nozzle 117.

As shown in FIG. 6, the cleaning brush 111 is brought into contact with the peripheral portion of the lower surface of the membrane 46, the outer circumferential surface of the membrane 46, and the lower surface and the inner circumferential surface of the retaining ring 42 in the presence of the cleaning liquid, thus removing the polishing liquid (slurry) attached to the membrane 46 and the retaining ring 42. Part of the cleaning brush 111 enters the gap between the membrane 46 for forming the pressure chambers and the retaining ring 42, thus removing the polishing liquid present in the gap. By providing such cleaning brush 111, it is possible to prevent the polishing liquid from being deposited and solidified and to prevent the subsequent wafer from being scratched. The polishing liquid attached to the cleaning brush 111 is removed by the cleaning liquid.

As shown in FIG. 8, a cleaning nozzle 128 for supplying a jet of a cleaning liquid to the gap between the membrane 46 and the retaining ring 42 to rinse away the polishing liquid (slurry) may be provided. The cleaning nozzle 128 has a vertical portion 128 b extending vertically, and an ejection mouth 128 c formed at the tip of the vertical portion 128 b. The cleaning nozzle 128 is configured so as to be movable between a cleaning position (indicated by solid lines) below the top ring 1 and a retreated position (indicated by dotted lines) lateral to the top ring 1. When the cleaning nozzle 128 is in the cleaning position, the ejection mouth 128 c is located below the gap between the membrane 46 and the retaining ring 42. In this position, the cleaning nozzle 128 supplies the jet of the cleaning liquid to the gap between the membrane 46 and the retaining ring 42 to remove the polishing liquid from the gap. Pure water may be used as the cleaning liquid.

The cleaning nozzle 128 is provided separately from the cleaning assembly 110, and is operated to clean the gap between the membrane 46 and the retaining ring 42 with the cleaning liquid before cleaning performed by the cleaning brush 111. Also, the cleaning liquid may be supplied to the gap between the membrane 46 and the retaining ring 42 from the cleaning nozzle 128 simultaneously with cleaning performed by the cleaning brush 111.

FIG. 9 is a side view showing another embodiment of the cleaning assembly 110. FIG. 10 is a top plan view of the cleaning assembly 110 shown in FIG. 9. In this embodiment, the brush arm 114 is divided into a first brush arm 114A and a second brush arm 114B, and the first brush arm 114A and the second brush arm 114B are fixed to each other by a first screw 131 and a second screw 132. The cleaning brush 111 is held by the first brush arm 114A, and the second brush arm 114B is connected through the link 124 to the air cylinder 121.

The first brush arm 114A is removably fixed to the second brush arm 114B by the first screw 131 and the second screw 132. Specifically, the first brush arm 114A and the cleaning brush 111 held by the first brush 114A can be separated from the second brush arm 114B by removing the first screw 131 and the second screw 132. Therefore, in the case where the cleaning brush 111 is worn away, the first brush arm 114A and the cleaning brush 111 can be easily replaced with new ones.

An elongated hole 134 is formed in the first brush arm 114A, and the second screw 132 is screwed into the second brush arm 114B through the elongated hole 134. When this second screw 132 is loosened, the first brush arm 114A can be rotated about the first screw 131 in a horizontal direction within a certain angular range. Therefore, as shown in FIG. 11, the relative position of the cleaning brush 111 held by the first brush arm 114A with respect to the second brush arm 114B can be changed.

The cleaning brush 111 extends vertically, however, as shown in FIG. 12, a tilt mechanism for tilting the cleaning brush 111 within a certain angular range with respect to the vertical direction may be provided.

FIG. 13 is a side view showing the cleaning assembly 110 according to still another embodiment. In this embodiment, the cleaning brush 111 extends obliquely toward the lower surface of the top ring 1. An upper end of the cleaning brush 111 forms a horizontal plane as with the above-described embodiment. More specifically, the bristles which constitute the cleaning brush 111 have different lengths depending on positions of the bristles. The bristle lengths on the retaining ring side are short and the bristle lengths on the membrane side (elastic membrane side) are long.

The cleaning liquid supply nozzle 117 is arranged to form a jet of the cleaning liquid horizontally and to apply the jet of the cleaning liquid onto the upper end of the cleaning brush 111. In this embodiment, the cleaning brush 111 is brought into contact with the peripheral portion of the lower surface and the outer circumferential surface of the membrane 46 and the lower surface of the retaining ring 42 to remove the polishing liquid from the gap between the membrane 46 and the retaining ring 42.

FIG. 14 is a top plan view showing still another embodiment of the cleaning assembly 110, and FIG. 15 is a side view showing the cleaning assembly 110 shown in FIG. 14. The configuration which will not be particularly described is the same as that of the above-described embodiment shown in FIGS. 5 and 6, and thus a duplicate description thereof will be omitted. In this embodiment, the cleaning assembly 110 includes a cleaning brush 140 which is brought into contact with the lower surface of the membrane 46.

Hereinafter, the above-described cleaning brush 111 is referred to as a first cleaning brush 111 and the cleaning brush 140 is referred to as a second cleaning brush 140.

The cleaning assembly 110 includes an extension arm 142 connected to the tip of the brush arm 114. The second cleaning brush 140 is held on an upper surface of the extension arm 142. The extension arm 142 is aligned with the brush arm 114.

As shown in FIGS. 16A and 16B, the first cleaning brush 111 and the second cleaning brush 140 are moved integrally by the cleaning brush movement mechanism 120. More specifically, when the piston rod 121 a of the air cylinder 121 moves forward, the brush arm 114 and the extension arm 142 are rotated by a predetermined angle, and thus the first cleaning brush 111 and the second cleaning brush 140 are moved to a cleaning position below the top ring 1 shown in FIG. 16A. When the piston rod 121 a of the air cylinder 121 moves backward, the brush arm 114 and the extension arm 142 are rotated in the opposite direction, and thus the first cleaning brush 111 and the second cleaning brush 140 are moved to a retreated position outside of the top ring 1 shown in FIG. 16B.

In the cleaning position shown in FIG. 16A, the first cleaning brush 111 is brought into contact with a peripheral portion of the lower surface of the membrane 46, an outer circumferential surface of the membrane 46, and a lower surface and an inner circumferential surface of the retaining ring 42. At the same time, as shown in FIG. 14, the second cleaning brush 140 is brought into contact with a zone more inner than the peripheral portion in the lower surface of the membrane 46. Therefore, while the first cleaning brush 111 removes the polishing liquid (slurry) from the gap between the membrane 46 and the retaining ring 42, the second cleaning brush 140 can remove the polishing liquid (slurry) from the lower surface of the membrane 46.

The above-described second cleaning brush 140 may be combined with another embodiment. FIG. 17 is a side view showing an embodiment in which the second cleaning brush 140 shown in FIGS. 14 and 15 is incorporated in the cleaning assembly 110 shown in FIG. 9. The second brush arm 114B is designed to be longer than the second brush arm 114B shown in FIG. 9 and has an extension portion, and the extension arm 142 is fixed to the extension portion of the second brush arm 114B. The second cleaning brush 140 is held on an upper surface of the extension arm 142.

FIG. 18 is a side view showing an embodiment in which the second cleaning brush 140 shown in FIGS. 14 and 15 is incorporated in the cleaning assembly 110 shown in FIG. 13. In this embodiment, the extension arm 142 is fixed to the tip of the brush arm 114. The second cleaning brush 140 is held on an upper surface of the extension arm 142.

In the embodiments shown in FIGS. 14 to 18, the first cleaning brush 111 and the second cleaning brush 140 are moved integrally by the cleaning brush movement mechanism 120. However, the second cleaning brush 140 may be provided in a separate location from the first cleaning brush 111, and another cleaning brush movement mechanism (not shown) different from the cleaning brush movement mechanism 120 may be provided to move the second cleaning brush 140 between a cleaning position and a retreated position.

The previous description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by limitation of the claims. 

What is claimed is:
 1. A polishing apparatus, comprising: a polishing table configured to support a polishing pad; a top ring which has an elastic membrane configured to form a pressure chamber for pressing a substrate against the polishing pad, and a retaining ring disposed around the elastic membrane and configured to press the polishing pad; a top ring rotating device configured to rotate the top ring about an axis of the top ring; and a cleaning brush configured to be brought into contact with a peripheral portion of a lower surface and an outer circumferential surface of the elastic membrane.
 2. The polishing apparatus according to claim 1, further comprising: a cleaning liquid supply nozzle configured to supply a jet of a cleaning liquid to the cleaning brush.
 3. The polishing apparatus according to claim 1, further comprising: a cleaning brush movement mechanism configured to move the cleaning brush between a cleaning position below the top ring and a standby position outside of the top ring.
 4. The polishing apparatus according to claim 1, wherein the cleaning brush is brought into contact with an inner circumferential surface of the retaining ring.
 5. The polishing apparatus according to claim 1, wherein the cleaning brush is brought into contact with a lower surface of the retaining ring.
 6. The polishing apparatus according to claim 1, wherein the cleaning brush is a first cleaning brush, and a second cleaning brush configured to be brought into contact with a zone more inner than the peripheral portion in the lower surface of the elastic membrane is further provided. 